A pseudo-intermediate tone process is known as a method of converting, in an image input/output system for outputting to an output apparatus such as a printer or a display multi-value image data read with an input apparatus such as a scanner or a digital camera, the multi-level (eg. 256 tones for an 8-bit accuracy) image data read with the input apparatus into image data having a number of tones which the output apparatus can output and pseudo-representing successive tones.
As this pseudo-intermediate tone process, a bi-level process is conventionally performed when the output apparatus can only represent a bi-level of ON/OFF of a dot. As this bi-level process, there are an error-diffusion process and a minimum-average error method that produce superior resolution and tone. The error-diffusion and minimum-average error methods which differ only in when to perform an error-diffusing task are logically equivalent.
Moreover, there is a multi-level error-diffusion process as one to which this error-diffusion process is applied to the number of tones of not only two levels but also three or more levels. Similar to the bi-level error-diffusion process, this multi-level error-diffusion process again produces superior tone and resolution.
On the other hand, various methods are known for securing the number of tones of three or more levels in the output apparatus. For example, in an inkjet recording-type image-forming apparatus for dispensing ink droplets so as to form an image, reproducing the number of tones of three levels or more is performed by such as: a method of controlling the amount of droplets dispensed, thereby varying a dot diameter as small, medium, or large; a method of overprinting dots; or a method of using a recording liquid ink of varying concentrations or a variable-density ink. It is noted that the concentration of a low-density ink is generally diluted to between ⅙ and ½ of a high-density ink.
Moreover, in an electrophotographic-type image-forming apparatus, pulse-width dividing light exposure in light-exposure writing so as to control the amount of light exposure that one dot forms, and varying the strength of a laser light used in light exposure so as to modulate the dot diameter, etc., are performed.
Now, it is known as one problem of the error-diffusion process as described above that a pseudo-contour R is produced as shown in FIG. 10 in proximity to a white-background portion (tone value 0) or a solidly-shaded portion (tone value 255) due to a delay in dot generation.
As described in Japanese Patent Laid-Open Publication 07-111591 (Patent document 1), it is known, for example, to vary a threshold value depending on the concentration and to eliminate a delay in dot generation at a highlighted portion or a delay in hole generation at a solidly-shaded portion in the bi-level error diffusion.
Moreover, in the multi-level error-diffusion process, there is also a problem of decreased image quality due to a delay in dot generation in a portion such that an N-level quantization output changes.
Then, as described in Japanese Patent Laid-Open Publication 2003-219161 (Patent document 2), there is an image-forming apparatus comprising quantizing means for quantizing to N values for each pixel of an M-tone image (where M>N) according to the error-diffusion process using N−1 reference threshold values, wherein the M tones are divided into N−1 intervals and a threshold value in each interval is varied depending on the input tone value of the pixel in question.
Moreover, as described in Japanese Patent Laid-Open Publication 2004-112089 (Patent document 3), a configuration is known for solving a problem of a pseudo-contour being produced in a case of uneven intervals of tones when the concentration in the vicinity of a tone (a quantization level) which an output apparatus has is output, the configuration comprising: means for outputting correction data in which an error diffused from already-quantized pixels therearound is added to multi-level image data of a pixel in question; means for setting a quantization-threshold value based on the multi-level image data of the pixel in question; means for comparing the correction data and the quantization-threshold value so as to output N-level image data; and means for calculating an error generated when generating the N-level image data. The configuration is set such that, assuming quantized values are 1, 2, . . . , a, b, . . . , N and tones of the quantized values are O1, O2, . . . , Oa, Ob, . . . , ON, an i-th threshold value Thi (i being an integer where 0<i≦N−1) in an interval between Oa and Ob of an input value, when an input concentration value is set to be In, is a value set based on Thi=Ki×In+(Oa+Ob)*(1−Ki)/2−(Oa−Oi), Ki being a real number which is greater than or equal to zero and being an integer where 0<i≦N−1.
However, while what is described in the Patent Document 1 solves a problem of an image distortion due to a delay in dot generation, it focuses attention on a single image or on an output result in a single error-diffusion process. Therefore, it does not address a problem of a pseudo-contour (a portion indicated with R in FIG. 11) being produced when adjoining and combining multiple error-diffused images or when dividing a single image into multiple areas so as to error-diffuse each of the areas.
Thus, there is a problem in that a pseudo-contour may be produced such as a portion indicated with R in FIG. 13 due to dots being generated too soon when the images are adjoined even when a delay in dot generation is improved such as a portion indicated with R in FIG. 12.
For a single image, dot generation being delayed or leading in an image-edge portion does not tend to be visually uncomfortable for humans. However, adjoining and combining such images together as described above causes a tendency of border portions to stand out and causes visual discomfort even with a small discontinuity, especially when both of the border portions are of the same tone.
Moreover, for such reasons as holding down the amount of memory used in the error-diffusion process, there is a case such that even a single image is divided into multiple small areas so as to error-diffuse for each of the image units. In such a case, it is likely that the same tone will continue in the process border so that a delay or a lead in dot generation is prone to becoming a problem.
There is a problem that in such a border at which the error-diffused image units adjoin, a stripe shape such that there is an empty gap (a portion indicated with R in FIG. 11) appears when there is a delay in dot generation while a dense stripe shape (a portion indicated with R in FIG. 13) appears when there is a lead in dot generation.